The Vernal (or Spring) Equinox is the more formal, scientific name for the First Day of Spring.
The equinox is the moment in which the plane of the Earth’s midline, the Equator, passes through the center of the Sun.
This creates the only time when the edge of night and day known as the “solar terminator” runs completely perpendicular to the Equator, causing the day and night to be equal in length.
In this moment, both the Northern and Southern hemispheres are also equally illuminated.
The only other equinox takes place in fall and is called the Autumnal Equinox. This year’s event takes place on September 22.